The need for short pulse (approximately <75 nanoseconds (ns)), high peak power (approximately 10 Kilowatt (kW)), eyesafe (approximately 1.4–1.7 micrometers (microns)) lasers has arisen in both military and commercial arenas. Scanning light detecting and ranging (LADAR) and target identification are examples of military applications needing such lasers, with free space communications likely the largest relevant commercial application.
Traditionally, sources for high peak power, eyesafe wavelength lasers have been wavelength-shifted Neodymium (Nd)-pumped wavelength shifters or Ytterbium (Yb)-sensitized Erbium (Er) lasers. The former has been successfully demonstrated with Nd-pumped Raman converters or optical parametric oscillators (OPOs) (with or without optical parametric amplifiers (OPAs)) but implementation has proven to be complex and costly. The latter has been the traditional approach for high peak power, eyesafe pulse generation at low to moderate (approximately 3 to 1000 Hertz (Hz)) pulse repetition frequencies (PRFs), but several problems in Yb-sensitized Er glass lasers limit gain needed to efficiently produce high peak power pulses at PRFs much greater than 1 kilohertz (kHz). These are: 1) Sufficient pump absorption requires roughly 10% Yb and hence 1% Er concentrations, bringing about Er upconversion which depopulates the upper lasing level, reducing gain and generating waste heat. 2) The energy transfer mechanism from Yb to Er represents an energy extraction bottleneck, since upon opening of a Q-switch much of the energy is stored in the Yb ions rather than Er ions. 3) With pump lasers at approximately 980 nm, the radiationless transition involved in the population of Erbium's upper lasing state (for 1.5 micron lasing) generates additional waste heat and renders the photon quantum efficiency near 63%; the latter limiting efficiency and the former tending to destabilize the laser resonator.
Recently, the advantages of resonantly pumping bulk crystals free of sensitizing dopants have been demonstrated at eyesafe wavelengths in actively Q-switched lasers. Resonant pumping enjoys several well-documented advantages as compared to the non-resonant pumping process used in sensitized Er:glass lasers. Higher efficiency and less waste heat follow from the greatly improved quantum efficiency. With no sensitizer present, all stored energy resides in inverted Er ions, and so more is available for extraction. Lastly no radiationless transition, with its parasitic waste heat, is necessary.
Due to the cavity dimensions associated with them, these resonantly pumped actively Q-switched Erbium lasers will struggle to produce pulse widths substantially less than 20 ns and still have peak powers greater than 1 kW. Hence, for example, they will remain inefficient in applications such as scanning LADAR, wherein resolution requirements prefer less than 3 ns pulse widths for efficient operation.
Conventionally, passively Q-switched eyesafe lasers have been non-resonantly-pumped, sensitizer-doped Er:glass lasers (bulk or fiber) operating near 1.5 microns. These typically produce pulse widths on the order of a few ns and peak powers up to ˜2 kW when operating at repetition rates near 1 kHz and up to tens of kW when operated near 10 Hz. Still many applications such as Scanning LADAR require tens of kW of peak power at 5 kHz and higher repetition rates. With its aforementioned inherent limitations of quantum defect, waste heat and energy storage bottleneck issues discussed above, sensitizer doped Er:glass passively Q-switch lasers have difficulty achieving these results.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.